Research On Resonant-type DC/DC Converters In DC Nanogrid

As the key facilities to interface different types of units and transmit power in DC nanogrids,power loss and dynamic characteristics of unidirectional and bidirectional DC/DC converters have great influences on the overall operation effciency and energy management of DC nanogrids.Among various DC/DC converters,the resonant-type DC/DC converters with high operation effciency and fast dynamic performances have the potential to be the interfaces between distinct units to realize the low consumption operation and flexible construction for DC nanogrids.The purpose of this paper is to select or propose the suitable resonant-type converters to achieve the interconnection between each unit and DC bus in typical DC nanogrid.The key technical problems of selected and proposed resonant-type converters are researched and solved to improve their practicability and stability.This paper analyzes the typical constructure of the DC nanogrid and classifies the existing DC/DC conveters for interconnection into five types according to differences of interconnected units,functions and power transmission direction.Thereinto,three unidirectional DC/DC converters for interconnecting the 380V DC bus and 48V DC bus,electric vehicle battery packs and distributed photovoltaic generation units can be replaced by LLC resonant DC/DC converters,meanwhile,two bidirectional DC/DC converters for the interconnections between the 380V DC bus and the 380V output of grid-connected rectifier and the energy storage equipments can be replaced by CLLC resonant DC/DC converters with variable-frequency control and LLC resonant DC/DC converters with the hybrid control of phase shift and variable frequency.The following five key problems for the above three kinds of resonant converters are studied and the solutions are given.1)Deadtime selection and calculation for LLC converters considering switching transient of MOSFET.The reasonable selection for deadtime is essential to achieve ZVS in a wide range of load and gain for LLC converters to have high efficiency.The methods for deadtime selection ignore the influence of MOSFET turn-off transient,so the selected deadtime need adjust when used and is not practical.The paper proposes the deadtime selection principles to achieve ZVS for wide-adjustable-range LLC converters considering the MOSFET turn-off transient.With the principles,LLC operation behaviors including the MOSFET turn-off transient under the worst-case conditions are described exactly and simplified properly.On the basis,minimum value of the required deadtime under the worst-case conditions for ZVS is calculated with the utilization of specification in the MOSFET datasheet.Then the selected deadtime can be abtained according to the deadtime selection principles.Experiment results verify the validity of the method.2)Research on optocoupler isolation of variable-frequency control chips for LLC converters.The periphery isolation circuit is necessary to achieve electrical isolation between the primary and secondary side of the LLC converter.However,when the traditional optocoupler isolation circuit is applied to variable-frequency control chips,the output current and parasitic capacitance of the optocoupler seriously impact and threaten the reliable operation for variable-frequency control chips.The traditional transformer and optocoupler isolation circuits for control chips are researched.The technical problems of the latter for LLC are pointed out.A novel optocoupler isolation circuit for the LLC variable-frequency control chips is proposed with part link of above two conventional isolation circuits and the selection and calculation method for its element parameters is given in detail.Simulation and experiment results verify the correctness of the new optocoupler isolation circuit and its parameter selection method.3)Research on operation modes for the CLLC resonant bidirection converter.Accurate description for operational behaviors and related characteristics of the CLLC converter is not only the theory basis but also the development source of CLLC.Traditional FHA only considers the fundamental component of voltage and current in CLLC and ignores the other high harmonic components,so the CLLC characteristic descriptions from FHA is inaccuracy and the CLLC various operation modes are hard to identify.In this paper,the main CLLC operation modes are identified and studied,moreover,the stages in first half switching cycle of CLLC modes are extracted and described in time domain.Then CLLC operation constraints are analyzed and obtained and the equations with voltage gain and other voltage and current variables unknown are established.CLLC behaviors and characteristics are able to be accurately described with numerical solutions of above equations.In addition,the boundary and distribution for CLLC modes are clarified to perfect the descriptions of the CLLC charactersitics and behaviors.Simulation results demonstrate that the theoretical results from CLLC operation mode analysis have high accuracy.4)Optimization design for resonant parameters of the CLLC resonant converter.The operation efficiency of the CLLC converter is closely related to its resonant network parameters.Conventional optimization design method for the CLLC resonant parameters is based on the low-accuracy formula of the CLLC voltage gain from FHA,thus the design result is hard to maximize the operation efficiency and may affect the CLLC operation stability.In the paper,the relationship between the ZVS condition of switch,condition loss,voltage gain characteristic and resonant elements are analyzed by using the accurate operation characteristic from the previous CLLC mode analysis.Then an optimum design method for CLLC resonant network parameters is proposed with considering the influence of resonant parameters on operational efficiency and closed-loop stability of CLLC.Experiment results show that the method can maximum the operation efficiency with the guarantee of closed-loop stability.5)Theoretical study and control implementation of a LLC resonant bidirectional converter baesd on hybrid control of variable frequency and phase shift.Existing resonant-type bidirectional converters as the interface between the bus and the energy storage equipment in DC nanogrid are hard ro achieve ZVS within the whole adjustable range and the control is complex.A novel LLC resonant bidirectional converter baesd on a hybrid control of variable frequency and phase shift is proposed.With ZVS for switches and ZCS for rectifiers in whole ranges of the voltage gain and transmission power,the novel converter has a high efficiency.Meanwhile,its control complexity is low owing to relying on one side switches to achieve the hybrid control.Operation behaviors and characteristics with the single variable frequency control or phase shift control are accurately described.Rationality of the hybrid control for LLC converter is demonstrated.A hybrid control scheme of variable frequency and phase shift with simple logic is proposed and easy to realize.The simulation and experiment results verify the correctness of the theoretical analysis for LLC and the effectiveness of the proposed hybrid control scheme with variable frequency and phase shift.The superiorities of the LLC resonant bidirection converter with hybrid control method in the operation efficiency and adjustable range are demonstrated.